The present method of chemical milling titanium is a batch process which produces a substantial amount of sludge and waste etchant which must be disposed of at a high cost and with a long downtime in order to make a fresh solution and to heat it to operating temperature.
Presently the chemical milling solution is discarded when the nitric acid concentration is depleted to below 8.0 normal or when the dissolved metal content of the solution reaches 30 to 40 g/l. At this point a portion of all of the etchant is discarded in order to make room for replenishment chemicals. In either case, the cost of chemicals needed to dissolve a pound of titanium is approximately the same.
When disposing of a depleted batch of etchant using the present batch process, 6360 gallons of depleted etchant is produced per 2200 lbs of dissolved titanium per batch. This volume must be neutralized, usually with calcium hydroxide, which increases the volume to 9381 gals. sludge needed to be disposed of. The current cost for removing and disposing of this amount of sludge is almost Seven Thousand Dollars for each 2200 lbs titanium dissolved. As will be explained in detail hereinafter, using the present continuous process, the cost of waste disposal is reduced to about Six Hundred Fifty Dollars per 2200 lbs of titanium dissolved, or less than 1/10 as much. This is in addition to the savings in make-up chemicals. Another savings results from the higher temperature (150.degree. F. versus 130.degree. F. for present processes) which results in evaporation of substantial amounts of water from the etchants. This means that it is not necessary to decant the etch solution when replenishing the activity content of the etchant.
We have found that in chemical milling of titanium and its alloys using conventional nitric acid and hydrofluoric acid etchants, a rock-like precipitate is formed after the etch solution becomes saturated. Using the etchant described in Coggins and Gumbelevicius U.S. Pat. No. 4,116,755, which includes a carbamide, such as urea, a water soluble precipitate forms when chemical milling is continued after the etchant is saturated with dissolved titanium.
In the conventional method, the solution is discarded when it becomes saturated to prevent excessive buildup of deposit in the etch tank, even though substantial amounts of unused acid (which must be neutralized for disposal) remain in the tank.
We are able to provide a continous process using a supersaturated etchant because the precipitate formed in our process can be dissolved in water for disposal. Thus, almost all of the etchant can be utilized and the bulk of solution is never discarded.
Accordingly it is a principal object of the present invention to provide a continuous process for chemical milling of titanium wherein the etchant is utilized at saturation point and is moved from one tank to another when a tank life has been utilized.
Another principal object of this invention is the continuous chemical milling of titanium utilizing a series of etch tanks wherein the cost of sludge removal is less than 1/10 of the normal cost and chemical costs are reduced through reuse of the etch solution and water loss by evaporation so that etchant does not need to be discarded to provide space for renewal of an etch tank.
Still another object of this invention is to provide a process whereby mostly water soluble sludge is produced during the chemical milling conditions and this, when solubilized and neutralized, produces titanium nitride which is saleable to other chemical processors.
A further object is to provide a unique and novel process for waste disposal in a titanium chemical milling process in which chemical make-up costs and sludge disposal costs are greatly reduced without impairing the efficiency of the etching process.
These and other objects and advantages will become apparent hereinafter.